Take a look at a more detailed look into the Canon Extender EF 1.4x III

Focal length extension for super-telephoto lenses

The Extender EF 1.4x III is specially designed to work in conjunction with L-series IS super-telephoto lenses, extending their focal length by a factor of 1.4x. The extender's design ensures there is no loss of lens performance or image quality.

High-quality construction

Improved anti-reflection materials inside the lens-barrel and a coating around the outer edges of the lens elements reduce flare. Construction is resistant to dust and water, helping to maintain images quality.

Improved communication between lens and camera

A built in microprocessor provides accurate transfer of data from between the lens and camera delivering an improved performance in AF and metering.

Compatible with L-Series telephoto and telephoto zoom lenses

The Extender EF 1.4x III is compatible with most telephoto and telephoto zoom lenses within Canon’s flagship professional L-series range, extending the focal length while only reducing the effective aperture of the lens by one stop.